Integrated channel filter using multiple resonant filters and method of operation

ABSTRACT

A circuit includes a first filter comprising a first inductor coupled to a first variable capacitor, wherein the first filter is associated with a first resonant frequency. The circuit further comprises an amplifier coupled to the first filter and a second filter coupled to the amplifier. The second filter comprises a second inductor coupled to a second variable capacitor, wherein the second filter is associated with a second resonant frequency that is substantially the same as the first resonant frequency. At least a portion of the first filter and at least a portion of the second filter are formed on an integrated circuit.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/107,167, filed Apr. 15, 2005 entitled “Integrated Channel FilterUsing Multiple Resonant Filters and Method of Operation,” now U.S. Pat.No. 7,304,533.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to signal processing, and moreparticularly to an integrated channel filter using multiple resonantfilters.

BACKGROUND OF THE INVENTION

It is desired to have a low noise amplifier (LNA) with a bandpass filterresponse and adjustable center frequency. A single resonant filter inthe emitter leg of the LNA is not adequate due to the presence of anadditional resonance caused by a collector load inductor and parasiticcapacitances. The resonance in the collector leg of the LNA does notnecessarily occur at the same frequency as the resonance in the emitterfilter, and therefore causes an undesirable broadening of the bandpassfilter response.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages and problemsassociated with prior amplifiers have been substantially reduced oreliminated.

In accordance with one embodiment of the present invention, a circuitincludes a first filter comprising a first inductor coupled to a firstvariable capacitor, wherein the first filter is associated with a firstresonant frequency. The circuit further comprises an amplifier coupledto the first filter and a second filter coupled to the amplifier. Thesecond filter comprises a second inductor coupled to a second variablecapacitor, wherein the second filter is associated with a secondresonant frequency that is substantially the same as the first resonantfrequency. At least a portion of the first filter and at least a portionof the second filter are formed on an integrated circuit.

The following technical advantages may be achieved by some, none, or allof the embodiments of the present invention.

Particular technical advantages of the present invention are achievedbecause the filters are formed at least in part on an integratedcircuit. For example, filters that are not formed on the integratedcircuit propagate the desired channels but reflect the undesiredchannels back to the transmitter or other source of the input signal.This reflection of undesired channels tends to corrupt all of thechannels in the input signal, including the desired channels. Thefilters which are formed on the integrated circuit communicate thedesired channels but do not reflect the undesired channels back to thetransmitter or source of the input signal. Instead, the undesiredchannels are dissipated in various components, such as the lossyelements, of the integrated circuit. The corruption of the desiredchannels is therefore no longer a significant issue.

By adjusting the capacitances of the two resonant filters, such thattheir resonant frequencies are aligned with each other, a sharperbandpass filter response is achieved. In effect, the quality factor ofthe filters and the amplifier are higher than alternative circuits withonly a single tunable resonant filter. This results in higher gain andgreater channel selectivity by the circuit.

These and other advantages, features, and objects of the presentinvention will be more readily understood in view of the followingdetailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of a system 10 that includes apre-select amplifier circuit and a tuner formed on an integratedcircuit;

FIG. 2 illustrates one embodiment of the pre-select amplifier circuitdepicted in FIG. 1;

FIGS. 3A-3B illustrate the bandpass filter responses of the filters inthe pre-select amplifier circuit depicted in FIG. 2; and

FIGS. 4A-4B illustrate variable capacitors used in the filters of thepre-select amplifier circuit of FIG. 2.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates one embodiment of a system 10 that includes apre-select amplifier circuit 12 coupled to a tuner 14. At least portionsof circuit 12 and tuner 14 are formed on an integrated circuit 16. Ingeneral, circuit 12 receives an input signal 20 comprising a pluralityof frequency channels. Circuit 12 filters and amplifies signal 20 forcommunication to tuner 14 as signal 22. Tuner 14 receives signal 22 andcommunicates output signal 24. In general, circuit 12 can achieve ahigher gain and greater channel selectivity of signal 20 because itincludes two resonant filter circuits 30 and 32 that are tuned tosubstantially the same resonant frequency.

Circuit 12 comprises first filter 30 and second filter 32 coupled toamplifier 34. Filters 30 and 32 comprise any suitable number andcombination of frequency selective components that may be formed onintegrated circuit 16. In a particular embodiment described in greaterdetail with reference to FIG. 2, filter 30 comprises a resonant filterthat includes an inductor and a parallel combination of capacitorsarranged in series with the inductor. Moreover, filter 32 comprises aresonant filter that includes an inductor and a parallel combination ofcapacitors arranged in parallel with the inductor. At least a portion ofthe capacitors may be switched in to or out of connection with theinductor of any given filter 30 or 32 to change the frequencyselectivity of the particular filter 30 or 32. These and other aspectsof circuit 12 are explained in greater detail with reference to FIG. 2.

Particular technical advantages of system 10 are achieved becausefilters 30 and 32 are formed at least in part on integrated circuit 16.For example, filters that are not formed on the integrated circuit 16propagate the desired channels of signal 20 but reflect the undesiredchannels of signal 20 back to the transmitter or other source of theinput signal 20. This reflection of undesired channels tends to corruptall the channels in the input signal 20, including the desired channels.Filters 30 and 32 formed on integrated circuit 16 communicate desiredchannels but do not reflect the undesired channels back to thetransmitter or source of input signal 20. Instead, the undesiredchannels are dissipated in various components, such as the loss elementsof integrated circuit 16. The corruption of the desired channels istherefore no longer a significant issue.

Tuner 14 comprises any suitable number and combination of active andpassive components including, but not limited to, gain control modules,mixers, and filters that may extract content from a desired radiofrequency spectrum and convert the content into a form that is usable,for example, by an access device. In one embodiment, tuner 14 comprisesa television tuner for use in a television system. Although circuit 12and tuner 14 are illustrated as separate components in FIG. 1, it shouldbe understood that in particular embodiments circuit 12 may be formedintegral to tuner 14. For example, circuit 12 may be formed integral toan input stage of tuner 14.

By arranging circuit 12 before or integral to an input stage of tuner14, system 10 achieves particular technical advantages. For example, thenumber of intermodulation products produced by the tuner 14 grows as thesquare of the number of channels that are processed by the tuner 14.Therefore, by filtering a number of the undesired channels from signal20 prior to the processing performed by tuner 14, circuit 10 eliminatesa large percentage of the intermodulation products produced by tuner 14.The range of gain programmability of tuner 14 is therefore increased.The reduction in intermodulation products also tends to reduce manysecond order intermodulation products (e.g., second order harmonicdistortion). Furthermore, as described above, the power and performancerequirements for tuner 14 are determined by the number of channelsprocessed by tuner 14. By reducing the number of channels processed bytuner 14, the power consumption and subsequent stages of tuner 14 isreduced.

Input signal 20 comprises a radio frequency signal. In a televisionsystem, signals representing individual channels are assigned tospecific frequencies in a defined frequency band. For example, in theUnited States, television signals are generally transmitted in a bandfrom 48 MHz to 852 MHz. In other countries, television signals aregenerally transmitted in a band from 470 MHz to 900 MHz.

In operation, circuit 12 receives an input signal 20 comprising a numberof channels. The desired tuning frequency of tuner 14 is determined.Based on that desired tuning frequency, the capacitor values for filter30 and filter 32 are set such that the resonant frequency of filter 32is substantially equal to the resonant frequency of filter 30 which issubstantially equal to the desired tuning frequency. As a result,circuit 12 filters input signal 20 to generate signal 22 having channelsin a desired frequency band. Moreover, amplifier 34 of circuit 12amplifies the channels of input signal 20 that are communicated insignal 22. Circuit 12 dissipates undesired channels in lossy elements ofintegrated circuit 16. Tuner 14 receives signal 22 and communicates anoutput signal 24 comprising one or more selected channels from thefrequency band associated with signal 22. In particular embodiments, theoutput signal 24 comprises a single channel in the television band.

FIG. 2 illustrates a particular embodiment of circuit 12 that includesfirst filter 30, amplifier 34, and second filter 32. Amplifier 34comprises transistor 50. Transistor 50 comprises a three terminaldevice. As illustrated, transistor 50 comprises an NPN transistor havinga base terminal that receives input signal 20, an emitter terminalcoupled to first filter 30, and a collector terminal coupled totransistor 52. First filter 30 comprises an inductor 54 having a firstlead coupled to the emitter of transistor 50 and a second lead coupledto variable capacitor 56. Variable capacitor 56 comprises a plurality ofcapacitors switchably coupled in parallel to each other and in serieswith inductor 54. Second filter 32 comprises inductor 60 having a firstlead coupled to a power source, V_(cc), and a second lead coupled totransistor 52. Second filter 32 further comprises a variable capacitor62 that comprises a plurality of capacitors switchably coupled inparallel to each other and in parallel with inductor 60. Second filter32 further comprises a resistor 64 coupled in parallel to inductor 60.Furthermore, second filter 32 comprises parasitic capacitancesrepresented by capacitor 66. Circuit 12 further comprises transistor 70which operates as an emitter follower coupled to the output of secondfilter 32 in order to isolate the output of second filter 32 fromcapacitive loading. Circuit 12 further comprises a current source 72. Anoutput terminal for signal 22 is coupled to the emitter of transistor70.

First filter 30 exhibits a first resonant frequency, f₁, given by thefollowing equation:

$f_{1} = \frac{1}{2\pi\sqrt{L_{1}C_{1}}}$

-   -   where:    -   L₁=inductor 54; and    -   C₁=variable capacitor 56.

Second filter 32 exhibits a second resonant frequency, f₂, given by thefollowing equation:

$f_{2} = \frac{1}{2\pi\sqrt{L_{2}( {C_{2} + C_{p}} )}}$

-   -   where:    -   L₂=inductor 60;    -   C₂=variable capacitor 62; and    -   C_(p)=parasitic capacitance 66.

As can be seen from the formulas above, first resonant frequency f₁ isbased upon the value of the first inductor 54 and the value of the firstvariable capacitor 56. In one embodiment, the value of first inductor 54is fixed and the first resonant frequency, f₁, is tuned by switching inor out one or more of the capacitors that make up variable capacitor 56.Again, this may be done in response to the desired tuning frequency asselected using tuner 14. The second resonant frequency, f₂, is basedupon not only second inductor 60 and second variable capacitor 62, butalso based upon parasitic capacitance 66. In general, second inductor 60and parasitic capacitance 66 are fixed and the value of second resonantfrequency f₂ is tuned by switching in or out one or more capacitors thatmake up variable capacitor 62. To operate circuit 12, the secondresonant frequency f₂ is tuned such that it substantially equals thefirst resonant frequency f₁. By adjusting the capacitances of the tworesonant filters 30 and 32 such that their resonant frequencies arealigned with each other, a sharper bandpass filter response is achieved,as illustrated with respect to FIGS. 3A and 3B. In effect, the qualityfactor of the circuit 12 is higher than alternative circuits with only asingle tunable resonant filter 30. This results in higher gain andgreater channel selectivity by circuit 12.

As can be seen by FIG. 2, circuit 12 combines a tunable resonant filter32 at the collector terminal of transistor 52 in addition to the tunableresonant filter 30 at the emitter terminal of transistor 50. A variablecapacitor 62 is placed within second resonant filter 32 such that theresonant frequency of the inductor 60, parasitic capacitance 66, andvariable capacitor 62 coincides with the resonant frequency of thetunable first resonant filter 30. In addition, the output of transistor50 is buffered from following stages in order to reduce the capacitiveload and also to provide a known capacitive load.

FIG. 3A illustrates an example bandpass filter response of a circuit 12that includes only first filter 30 and amplifier 34. The graph of FIG.3A plots frequency along the x-axis and gain (e.g., V_(out)/V_(in))along the y-axis. FIG. 3B illustrates an example bandpass filterresponse of a circuit 12 that includes amplifier 34 coupled to bothfirst resonant filter 30 and second resonant filter 32, as depicted inFIGS. 1 and 2. As with FIG. 3A, FIG. 3B plots frequency along the x-axisand gain along the y-axis. As can be seen by the bandpass filterresponse of FIG. 3B as compared to the bandpass filter response of FIG.3A, a sharper bandpass filter response with higher gain is achievableusing first and second resonant filters 30 and 32 as opposed to a singleresonant filter 30. In particular, by adjusting the capacitances ofresonant filters 30 and 32 such that their resonant frequencies arealigned with each other, a sharper bandpass filter response is achieved.This results in higher gain and greater channel selectivity by circuit12. The filter responses of circuit 12 may vary from the example filterresponses illustrated in FIGS. 3A-3B without departing from the scope ofthis disclosure.

FIGS. 4A-4B illustrate particular embodiments of variable capacitors 56and 62, respectively. Referring to FIG. 4A, variable capacitor 56comprises a parallel combination of capacitors 56 a-56 n switchablycoupled to each other. Variable capacitor 56 is coupled in series toinductor 54. In particular, a second capacitor 56 b is switchablycoupled in parallel to first capacitor 56 a. A third capacitor 56 c isswitchably coupled in parallel to first capacitor 56 a. An n^(th)capacitor 56 n is switchably coupled in parallel to first capacitor 56a. Therefore, signals associated with capacitors 56 b through 56 n areswitchably coupled to a signal associated with capacitor 56 a. Theselected combination of capacitors, 56 a through 56 n, may be referredto collectively as capacitors 56 and generally as variable capacitor 56.Capacitors 56 each have a first terminal coupled to a terminal ofinductor 54. First capacitor 56 a has a second terminal coupled toground. A first switch 55 a shorts inductor 54 to ground. A secondswitch 55 b couples the second terminal of second capacitor 56 b to thesecond terminal of first capacitor 56 a. A third switch 55 c couples thesecond terminal of third capacitor 56 c to the second terminal of firstcapacitor 56 a. Switches 55 are selectively enabled based upon a commandsignal received by circuit 12.

Referring to FIG. 4B, variable capacitor 62 comprises a parallelcombination of capacitors 62 a-62 n switchably coupled to each other.Variable capacitor 62 is coupled in parallel to inductor 60. Inparticular, a second capacitor 62 b is switchably coupled in parallel tofirst capacitor 62 a. A third capacitor 62 c is switchably coupled inparallel to first capacitor 62 a. An n^(th) capacitor 62 n is switchablycoupled in parallel to first capacitor 62 a. Therefore, signalsassociated with capacitors 62 b through 62 n are switchably coupled to asignal associated with capacitor 62 a. The selected combination ofcapacitors, 62 a through 62 n, may be referred to collectively ascapacitors 62 and generally as variable capacitor 62. Capacitors 62 eachhave a first terminal coupled to a terminal of inductor 60. Firstcapacitor 62 a has a second terminal coupled to ground. A first switch65 a shorts inductor 60 to ground. A second switch 65 b couples thesecond terminal of second capacitor 62 b to the second terminal of firstcapacitor 62 a. A third switch 65 c couples the second terminal of thirdcapacitor 62 c to the second terminal of first capacitor 62 a. Switches65 are selectively enabled based upon a command signal received bycircuit 12.

The components of variable capacitors 56 and 62 are formed on integratedcircuit 16. The values of capacitors 56 and 62 may be selected within aparticular filter 30 or 32 such that the appropriate combinations ofcapacitors 56 and 62 within any given filter 30 or 32 coupled in serieswith inductor 54 or in parallel with inductor 60, respectively, provideappropriate resonant frequencies and, therefore, bandpass filteringabout appropriate center frequencies. For example, the resonantfrequency for filter 32 will depend not only upon the value of thecapacitors 62, but also upon the value of parasitic capacitances 66 andinductor 60. As a result, the values of capacitors 56 may or may not bethe same as the values of capacitors 62. Although FIGS. 4A-4B illustratevariable capacitors 56 and 62 having particular numbers and arrangementsof capacitors, it should be understood that any given variable capacitor56 and 62 may have any suitable number and arrangement of capacitors toderive a desired level of granularity associated with the ranges offrequency selection.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade hereto without departing from the scope of the invention as definedby the appended claims.

1. A circuit, comprising: a tuner comprising a tuning circuit; and apre-select amplifier circuit communicatively coupled to the tuningcircuit and comprising: a first filter separate from the tuning circuitof the tuner, and comprising a first inductor coupled to a firstvariable capacitor, wherein the first filter is associated with a firstresonant frequency; an amplifier coupled to the first filter andseparate from the tuning circuit of the tuner; a second filter coupledto the amplifier, separate from the tuning circuit of the tuner, andcomprising a second inductor coupled to a second variable capacitor,wherein the second filter is associated with a second resonant frequencythat is substantially the same as the first resonant frequency; and anemitter follower circuit coupled to the second filter and operable tobuffer the output of the second filter from capacitive loading.
 2. Thecircuit of claim 1, wherein the first variable capacitor comprises aplurality of capacitors switchably coupled to each other, each capacitorhaving a value that is selected to achieve a particular first resonantfrequency.
 3. The circuit of claim 1, wherein the second variablecapacitor comprises a plurality of capacitors switchably coupled to eachother, each capacitor having a value that is selected to achieve aparticular second resonant frequency.
 4. The circuit of claim 3, whereinthe second variable capacitor further comprises parasitic capacitancesthat contribute to the second resonant frequency.
 5. The circuit ofclaim 1, wherein the first resonant frequency is based at least in partupon the value of the first inductor and the value of the first variablecapacitor.
 6. The circuit of claim 1, wherein the second resonantfrequency is based at least in part upon the value of the secondinductor and the value of the second variable capacitor.
 7. The circuitof claim 1, wherein the first resonant frequency and the second resonantfrequency are determined according to a desired tuning frequency.
 8. Thecircuit of claim 1, wherein the circuit is operable to receive a signalhaving a frequency in the range from 470 MHz to 900 MHz.
 9. The circuitof claim 1, wherein the pre-select amplifier circuit comprises an inputstage of the tuner that is separate from the tuning circuit of thetuner.
 10. The circuit of claim 1, wherein the circuit is operable toreceive a signal having a frequency in the range from 470 MHz to 900MHz.
 11. A circuit, comprising: a first filter comprising a firstinductor coupled to a first variable capacitor, wherein the first filteris associated with a first resonant frequency; an amplifier coupled tothe first filter; a second filter coupled to the amplifier andcomprising a second inductor coupled to a second variable capacitor,wherein the second filter is associated with a second resonant frequencythat is substantially the same as the first resonant frequency; anemitter follower circuit coupled to the second filter and operable tobuffer the output of the second filter from capacitive loading; and atuner communicatively coupled to the emitter follower circuit.
 12. Thecircuit of claim 11, wherein the first variable capacitor comprises aplurality of capacitors switchably coupled to each other, each capacitorhaving a value that is selected to achieve a particular first resonantfrequency.
 13. The circuit of claim 11, wherein the second variablecapacitor comprises a plurality of capacitors switchably coupled to eachother, each capacitor having a value that is selected to achieve aparticular second resonant frequency.
 14. The circuit of claim 13,wherein the second variable capacitor further comprises parasiticcapacitances that contribute to the second resonant frequency.
 15. Thecircuit of claim 11, wherein the first resonant frequency is based atleast in part upon the value of the first inductor and the value of thefirst variable capacitor.
 16. The circuit of claim 11, wherein thesecond resonant frequency is based at least in part upon the value ofthe second inductor and the value of the second variable capacitor. 17.The circuit of claim 11, wherein the first resonant frequency and thesecond resonant frequency are determined according to a desired tuningfrequency.